51

Study on Applicability of Electric Sounding forInterpretation of the Internal Structure of Glacial Moraines
Kengo OHASHI1*, Toru KOIKE1 Shuhei TAKENAKA1 and Jun UMEMURA2
1

Earth System Science Co., Ltd.,
1-23-1, Shinjuku, Shinjuku-ku, Tokyo 160-0022, Japan
2
College of Engineering, Nihon University,
Tamura-machi Tokusada, Koriyama 963-8642, Japan*e-mail: ohashi@ess-jpn.co.jp

Abstract
The internal structure of moraine dams, especially the distribution of core ice and bedrock, is one of the
key factors in assessing GLOF risk. In this study, we discuss the interpretation of the results of 2D electricsounding at three moraines in northern central Bhutan from the standpoint of not only their specific resistivity
values, but also distribution patterns, in conjunction with field observations and previous studies. We foundthat:
1) Zones of specific resistivity exceeding 100,000 ohm-m could be regarded as massive dead ice, whilezones of 10,000 - 50,000 ohm-m were assumed to be a mixture of fragmented ice and rock debris, where
the higher the value was, the higher the ice content.
2) Resistivity values of rock debris were 5,000 - 10,000 ohm-m depending on the water content of matrix
materials and degree of freezing.
3) Resistivity values of bedrock overlapped those of rock debris in the moraine. However, in the case of thestudy area, some distinctive patterns of lower resistivity, such as steep angle zones that were concordant
with lithological features of the bedrock of the area, were used to identify bedrock.
Further studies employing electric sounding accompanied by other exploration methods are still necessary to improve the accuracy of the interpretation.
Key words: dead ice, GLOFs, moraine dam, resistivity profile, two-dimensional electric sounding

1. Introduction
Triggers and mechanisms of glacial lake outburst
floods (GLOFs) have been a controversial issue. Many
previous studies have revealed that GLOFs are inducedby sudden increased overtopping flows over the moraine
dam, which are related to calving and/or mass rock failure from the edge of the glacial body and lakeside deposits into the lake (e.g., Iwata et al., 2002). It is reported
that gradual subsidence of the moraine surface as a resultof melting of dead ice in the moraine contributes to thestability of the moraine dam with a lower lake level(Takenaka et al., 2010), whereas rapid melting of deadice may promote topographical instability of the moraine
dam. In any scenario, it is highly necessary to obtainsignificant information on the internal structure andphysical and geotechnical properties of the moraine damin order to assess the breach potential of the dammedlake.
Owing to severe difficulties in accessibility of highaltitude regions in the Himalayas, no systematical or
Global Environmental Research
16/2012: 51-58
printed in Japan

comprehensive investigations including drilling surveys
in moraine dams have been conducted until now.
By contrast, geophysical exploration methods which
are relatively portable have been commonly applied forclarifying the internal structures of moraine dams. In
particular, the electric sounding method has been employed in many previous studies because of prompt
operation for data acquisition even over a broad exploration area, and its adaptive flexibility to changes in terrain
conditions (e.g., Hauck et al., 2003; Gibas et al., 2005).
On the other hand, specific resistivity values of moraine
substances obtained by electric sounding depend on not
only their original electric character but also the condition of the water content, quality, temperature, etc.
Therefore, a comprehensive interpretation of resistivity
values is essential for identifying the materials.
In this study, we carried out two-dimensional (2D)electric sounding at one debris-covered glacial moraine
and two glacial lake moraine dams in northern central
Bhutan for the purpose of verifying specific resistivity
values indicating constituent materials such as dead ice,©2012 AIRIES52

K. OHASHI et al.

debris and bedrock. In addition, the interpretation of“distribution patterns” on resistivity profiles is discussed
in consideration of the results of previous studies as well.

2. Study Area
The target glacial moraines of the Tsho Jo Glacier,Lake Zanam C (tentatively named by Komori et al.,2010) and Lake Metatshota are located in the High
Himalayan region of northern central Bhutan as shown in
Fig. 1. The Tsho Jo Glacier, a debris-covered glacier witha total length of 13 km and an average width of 1.7 km, is
one of the largest glaciers in Bhutan. A sudden abnormal
flood originating from the terminus of glacier wasobserved on 29 April 2009. It was identified as a discharge from a supra-glacial lake through an internal
channel in the glacier (Komori et al., 2012, in this issue).
Due to this sudden discharge, a large amount of soil anddebris from the terminal moraine was flushed away, andthereby an ice body 5 m in height was exposed in thedebris cliff near the outlet. 2D electric sounding was
carried out in order to calibrate specific resistivity values
between the ice and the surrounding debris materials.
Lake Zanam C is located in a small tributary of Sacha
Chu River, a branch of the Mangde Chu River. The lakesize is rather small: 0.9 km long by 0.2 km wide. In our
preliminary geomorphological study, the breach potential
was presumed to be the highest in the Mangde Chu river
basin because of the 200 m steep slope of the moraine
dam.
Lake Metatshota, the largest glacial lake in the
Mangde Chu river basin with a surface area of 1.34 km2,

is ranked as one of 23 potentially dangerous glacial lakes
in Bhutan (Mool et al., 2001). Undulating morainealignments encompassing the frontal lake have alreadybeen deeply eroded by the outlet flow. The present
relative height of the moraine dam is only 25 m from thefrontal plain. Thus it seems to be stable for breachingcompared with Lake Zanam C.

3. Method of Exploration
Expeditions were conducted on 5 September 2010
and 11 October 2011. A McOHM Profiler-4 (OYO
Corporation, Japan) was utilized as the electric sounding
machine. This model includes an Operation System withan SSD drive in its body; therefore, the user can monitor
the real-time electric potential wave and decay curve
during measurement. It provides a significant advantage
in measuring by enabling the user to review abnormal
values in case extremely high ground resistances are
predicted. The specifications of the utilized equipment
are shown in Table 1.
Survey lines and electrode intervals of 2.5 m or 5.0 m
were applied according to the depth to be sounded at eachobjective site. Several electrode alignments of pole-pole,
pole-dipole and dipole-dipole were examined to clearly
represent the resistivity profile in each moraine. In orderto reduce the high ground electric resistance of debrisrich moraine surfaces, multiple iron poles were inserted
into one electrode position. In addition, a bentonite
solution with highly conductive fabric was buried into
the ground.

Fig. 1 Location of the target moraines in the study area.

Study on Applicability of Electric Sounding for Interpretation of the Internal Structure of Glacial Moraines
Table 1

4. Results

Electric sounding equipment utilized in this study.
All items were produced by OYO Corporation.

Multi-channel
Digital Res istivity M eter:
McOHM Profiler-4 XPE 2140D
Electrode Switching Sy stem :Scanner 64 2141
Boos ter:Power Booster 2142
Analy sis Software:

4.1 Tsho Jo Glacier moraine
The debris cliff formed by the abnormal discharge in2009 at the terminus of the Tsho Jo Glacier, has receded
50 m since September 2009. The outcrop of the ice bodyis visible only at the end of the retreating cliff, where theoutlet flow is rapidly eroding the debris materials, as
shown in Fig. 2. It is unclear whether the ice body is apart of the glacial ice lying under the moraine or just an
ice fragment in the debris deposit. Electric sounding was
carried out on the moraine surface near the outcrop of theice body. The survey line was laid out for a length of 150
m parallel to the ice cliff, as shown in Fig. 3.

Specifications

Item

Output Method:

Alternative DC

Output Voltage:

Maximum 400V

Output Current:

Max 120mA

Max Electrode:64 electrodes

Output Voltage:

Maximum 200V

Output Current:

250,500,750,1000mA

ElecImager / 2D

OutletSurvey Line

Fig. 2 Overview of survey area at the Tsho Jo Glacier moraine. The base view isportrayed by an ALOS-PRISM elevation model and panchromatic image.

Survey Line0158

Ice Outcrop
Outlet(m) 160

140

100

120

80

40

60

20

0

< 10,000
20,000 - 50,000

Ice Outcrop
OutletOhm-m
Ohm-m
100,000

20
50,000Debris> 100,000
10,00040(m)

Coexistnace ofDebris & Ice (fragment?)

53

Massive
Dead Ice

5,000

0Fig. 3 Survey line and result of electric sounding at the Tsho Jo Glacier moraine.

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K. OHASHI et al.

Two masses of high-resistivity zones showing over
100,000 ohm-m were observed 10 m below ground level.
Relatively lower resistivity of 20,000 - 50,000 ohm-m at
5 - 10 m below ground level corresponds to debris andthe ice body observed in the cliff. The current outlet was
located between the two zones. The shallow area above
5 m below ground level basically showed a resistivity
value of less than 10,000 ohm-m, as shown in Fig. 3.
Consequently, the moraine materials of the Tsho Jo
Glacier can be divided into three layers, comprising
1) a zone of more than 100,000 ohm-m, 2) a zone with arange of 20,000 - 50,000 ohm-m in debris with ice, and
3) a surface zone of one-order below.

4.2 Lake Zanam C moraine dam
The moraine dam of Lake Zanam C takes the shape ofa steep slope around 200 m high from the Sacha Chu
riverbed. We set our survey line on the crest of the terminal moraine. The electric sounding was carried out
using pole-dipole alignment at 5 m intervals. The location of the survey line and the results are shown in Fig. 4.
Resistivity in the moraine dam is limited to values ofless than 15,000 ohm-m. There is no high resistivity zone
indicating dead ice until 80 m depth from the groundsurface. The highest resistivity zone was observed in thedown slope of the moraine dam, where a resistivity zoneof 5,000 - 50,000 ohm-m is distributed at 40 m thickness.

0270

Survey Line(m) 260

240

220

200

180

200m

160

140

120

100

80

60

40

20

0

5,000 - 15,000< 5,000

Ohm-m
100,000

20

Seepage40

50,00060

Debris(saturated)

80

Bedrock withfracture zone

10,000Debris(less matrix)

100

5,000

0

(m)

Fig. 4 Survey location and result of electric sounding at the Lake Zanam C
moraine dam. The base view is portrayed by an ALOS-PRISM elevation
model and panchromatic image (blank arrows indicate the cross-section
line in Fig. 5).
(m)

(m)

Fig. 5 Assumed geological section of Lake Zanam C(horizontal scale : vertical scale = 2:1).

Study on Applicability of Electric Sounding for Interpretation of the Internal Structure of Glacial Moraines

55

Outlet0155(m) 140

0

120

100

80

25m

Survey Line

60

40

20

0

5,000 - 10,000
Ohm-m
100,000

20

Debris(less matrix)

Debris(saturated)
2,000 - 5,000

50,00010,000

40
5,000

0

60 (m)

Fig. 6 Survey location and result of electric sounding at the Lake Metatshota
moraine dam. The base view is portrayed by an ALOS-PRISM elevation
model and pan-sharpened image

This zone may be composed of less matrix debris deposits according to the field observation. On the other hand,a resistivity zone of less than 5,000 ohm-m near thelakeside indicates saturated debris materials. Though theresistivity value gradually increases toward the deeper
portion, several lower resistivity zones with steep angles
are observed below 25 m from the ground level, as shownin Fig. 4. As discussed later, this steep-angle structure canbe interpreted as fracture zone bedrock. A cross section
shown in Fig. 5 indicates that the bedrock elevation at themoraine dam is concordant with the deepest level of thelake bathymetry reported by Komori et al. (2010). The
bedrock elevation is 150 m higher than the riverbed of the
Sacha Chu River.
4.3 Lake Metatshota moraine dam
Electric sounding was carried out on the narrow flatplan just behind the highest moraine hill, as shown in
Fig. 6. The surface around the survey line is composed offine sand and silt with high water content that may haveless ground resistance for better sounding accuracy.
As the results in Fig. 6 show, no high resistivity zone
was observed in any part of the moraine until 50 m below
the ground level. High resistivity values ranging from
5,000 - 10,000 ohm-m were obtained at the end point of
the survey line, where less matrix debris deposits are
distributed. The saturated zone below the lake water levelshows less than 5,000 ohm-m and decreases toward the
deep direction. Since the resistivity distribution patternshows a horizontal structure, no steep angle structure isobserved, unlike at Lake Zanam C. Such a horizontal
pattern is commonly observed in sediments. In the Lake

Metatsota moraine dam, therefore, no bedrock can be
expected to exist until deeper than 50 m below the groundlevel.

5. Discussion
Inevitably each material has a specific range of electric resistivity values and it must be comprehensively
verified by field observations in consideration of previous studies. The resistivity value range of the moraine
dam materials can be summarized as shown in Fig. 7,
using data obtained from previous studies and this study.
5.1 Interpretation from resistivity values anddistribution patterns
5.1.1 Resistivity value of ice
Most studies have identified high resistivity zones
showing more than 100,000 ohm-m as underlying deadice bodies in the moraine (e.g., Hauck et al., 2003). In
this study, in the Tsho Jo Glacier moraine, likewise, alarge zone of 100,000 ohm-m below 10 m from theground level was regarded as dead ice. On the other hand,it can be clarified that the layer of ice-containing debris
material showed relatively lower values of 20,000 50,000 ohm-m. Therefore, it is assumed that the zone
indicating resistivity values of more than 20,000 ohm-m
may contain ice which is either massive or fragmented indebris material conditions. In the case that a resistivity
zone of 10,000 - 20,000 ohm-m appears in the deep
potion of a moraine, it is also expected to involve someamount of ice fragments.

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K. OHASHI et al.

5.1.2 Resistivity values of debris
Resistivity values of debris deposits vary depending
highly on the water content of the matrix substances andthe frozen condition. The results at Lake Zanam C andLake Metatshota in this study suggest that the debris withless matrix or coarse sandy materials near the groundsurface shows 10,000 ohm-m, whereas the saturated zone
near the outlet or lakeside shows less than 5,000 ohm-m.
5.1.3 Resistivity values of bedrock
As the resistivity range of bedrock overlaps with thatof debris materials in the moraine, it is difficult to identify the boundary between them with any definitivity. In
case of the Lake Zanam C moraine dam, a resistivity
distribution pattern with several steep angle zones oflower resistivity was observed. In general, such verticalstructures are often seen in fracture zones of bedrocksuch as shear zones or joint systems. Indeed, the lithology of the study area is composed of granitic gneiss withthe predominant structure dipping toward the north,which is concordant with the observed resistivity patternin the moraine. Such an approach to interpretation canhelp to identify materials in case there are no distinct

electric differences. However, it may not be applicable tomassive rock without fractures or predominant joint
systems.
5.2 Applicability of interpretation
Consequently, the proposed procedure to identify theinternal structure of moraine dams is summarized inTable 2 in consideration of resistivity ranges and distribution patterns. In order to generalize this classification
method to make it applicable to other regions, the following issues must be considered.
1) Reducing the ground electric resistance of debrisrich moraine surfaces is a focal issue for securing
accuracy of electric sounding, especially for scanning
distribution patterns of resistivity. Using bentonite orelectrolyte fluid and multiple electrodes is recommended
as one practical solution for electric sounding.
2) Interpretation using the inter-structure of bedrock
cannot be applied in the case that the survey line is lying
parallel to the bedrock structure. If the existence of
bedrock is suspected, the survey line must be extended
and/or multiple survey lines added.

Fig. 7 Ranges of resistivity values of moraine dam materials.
Table 2 Flow chart for interpreting internal structure using resistivity ranges and distribution patterns.
Res istiv ity Value< 10,000 ohm-m
10,000 - 20,000 ohm-m
20,000 - 100,000 ohm-m> 100,000 ohm-m

Dis tribution Pattern

M aterialwith Vertical Structure

Bedrock

Horizontal or Irregular Structure⇒

Limited at shallow zone
Not limited at sallow zone
Irregular Distribution
Extensive and Mass block

Debris⇒

Debris with Ice Fragments
Saturated Dead Ice ?
Dead IceStudy on Applicability of Electric Sounding for Interpretation of the Internal Structure of Glacial Moraines

3) The depth limitation of 2D electric sounding isapproximately 100 m in terms of securing sound data
accuracy. When bedrock is expected in a deeper portion,
TEM (Transient Electro-Magnetic method) or combination with appropriate physical exploration methods
should be employed as supplemental methods. In thisstudy, the micro-tremor array method was also carriedout at each site. However, no dependable results could beobtained due to insufficient micro-tremor at the sites.

6. Conclusion
In terms of GLOF risk assessment, an important issue
is to understand moraine dam internal structures, especially the distribution of huge dead-ice bodies andbedrock, which may have an effect on the breach potential and total flood volume. In this study, we carried out
2D electric sounding at three moraines in northern central
Bhutan. We also interpreted the appearance and distribution of dead ice, debris and bedrock, considering specificresistivity values and their distribution patterns.
Electric sounding is one of the most popular andapplicable methods of analyzing the internal structures ofglacial moraines. Securing accuracy in sounding data
could make a large contribution to the understanding of
not only internal structures but also the breach potentials
of moraine dams. Further empirical studies in conjunction with other physical exploration methods are highly
anticipated in order to reveal the electric features ofmoraine materials based on both resistivity values andtheir distribution patterns.

Acknowledgements
We thank all the Bhutanese staff of Department of
Geology & Mines for their great assistance and hospitality during the expedition into the high mountain area.
Special thanks to our medical advisor, Mr. Kinley Dorji,
for carefully nursing us through trouble. We are gratefulto the members of the science workshop at Hokkaido
University for their valuable counsel and discussion. This
study was carried out as a part of the “Study on Glacial
Lake Outburst Foods in Bhutan Himalaya” project under
SATREPS supported by JST and JICA.
References
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Skuk, M. Meyer and C. Janda, E. Burgschwaiger (2000) Final
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University of Viena, Austria.
Hauck, C., M. Vonder and D. MÜHLL (2003) Evaluation of geophysical techniques for application in mountain permafrost
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Ikeda, A. (2006) Combination of conventional geophysicalmethods for sounding the composition of rock glaciers in the
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Iwata, S., Y. Ageta, N. Naito, A. Sakai, C. Narama and Karma(2002) Glacial lakes and their outburst flood assessment in the
Bhutan Himalaya. Global Environmental Research, 6: 3-17.
Komori, J., T. Phuntsho and S. Takenaka (2010) Bathymetric
Topgraphy of Glacial Lake in Bhutan Himalaya. Abstracts ofJapan Geosciences Union Meeting 2010.
Komori, J., T. Koike, T. Yamanokuchi and P. Tshering (2012)
Glacial lake outburst events in the Bhutan Himalayas, Global
Environmental Research, 16: 59-70. (in this issue)
Mool, P. K., D. Wanda, S. R. Bajracharya, K. Kunzang and S. P.
Joshi (2001) Inventory of Glaciers, Glacial Lakes and Glacial
Lake Outburst Floods. Monitoring and Early Warning Systems
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Takenaka, S., T. Yabuta and H. Fukui (2010) Distribution of deadice inside moraine dam body of Imja Glacial Lake, Khumbu
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Kengo OHASHI
Kengo OHASHI is a consultant specializing in
geophysical engineering, at Department ofEnvironmental & Disaster Management at Earth
System Science Co., Ltd. His fields of study
include geophysical exploration for water resource
development, landslide stability and constructionof infrastructure. Groundwater development in arid regions of Africancounties by use of remote sensing and 2D / 3D electrical sounding is one ofhis main focuses these days.

Toru KOIKE
Toru KOIKE is a Deputy General Manager of theEnvironmental & Disaster Management Department at Earth System Science Co., Ltd. He has
been working in the field of disaster management,
including geological and river engineering under
Official Development Assistance funding from theJapanese government. During 2000-2002, he lived in Bhutan and worked
for the Department of Geology & Mines. Currently he is working indisaster related projects in Thailand, Vietnam, Bhutan, Bolivia andCaribbean countries.

Shuhei TAKENAKA
Shuhei TAKENAKA is dedicated staff of Earth
System Science Co., Ltd., for the JICA/JST
Bhutan GLOFs project. Although he got his
M.Sc. in Glaciology from Nagoya University 30
years ago, prior to joining the project, he worked
as a manager at an electronic device company.
Some five years ago, he decided to return from “cyber space” to “the realworld” looking for a chance to perform field work and more directly
contribute to the human community. On Himalayan trails, he feels happy
with his decision despite the aches in his back and knees. In addition to
glaciers, he is interested in how local communities in remote areas arebeing changed by modern technologies like TVs and cell phones.

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K. OHASHI et al.
Jun UMEMURA

Jun UMEMURA is a lecturer at the College ofEngineering, Nihon University. He also is amember of the technical committee for waste
disposal and conservation of the environment ofFukushima Prefecture. His speciality is geotechnical engineering and engineering geology,
especially the prevention of landslide disasters. In 2001, he was studying at
Tribhuvan University, Nepal; and in 2002, at the Swiss Federal Institute ofTechnology, Zurich. Recently his endeavors have focused on many of the
major disasters in Japan, including those caused by the 2008 Iwate-Miyagi
Nairiku Earthquake and the 2011 Tohoku Earthquake off the Pacific coast.

(Received 26 December 2011, Accepted 13 February 2012)